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An early gravitational dance made the giant planets tilt, an astronomer suggests.

The shift probably happened billions of years ago when the bigger planets in our solar system were closer together than they are now, and the gravity of each one pulled on the others, writes Argentinian researcher Dr Adrian Brunini today in the journal Nature.

This "neutral gravitational interaction" caused Jupiter, Saturn, Uranus and Neptune to have tilted axes that were determined as they moved through the solar system to take their current positions far from the Sun, says Brunini, from the Facultad de Ciencias Astronomicas y Geofisicas in Buenos Aires.

This is a departure from an earlier theory that holds that the massive planets' tilts, or obliquities, were caused by collisions with Earth-sized space rocks during the early period of the solar system.

"This model has some problems that were not clear how to solve," Brunini says. "For example, we believe that such a big object never existed in the outer solar system."

Gravity on gravity

Brunini used numerical models to show that the outer planets' obliquities could have been created by gravitational interactions.

All the planets in our solar system have tilted axes but the bigger ones have axes that lean at a constant angle, while the smaller ones like Earth have obliquities that can change.

Despite the potential for change, Earth's axis has been leaning at about 23° for millions of years and is almost completely stabilised by the Moon's gravitational pull, Brunini says.

But Mars' axis might change over tens of millions of years.

Earth's tilt stable but crucial

For humans, the reliability of Earth's tilted axis is important as it is responsible for the change of seasons. At the point in its annual orbit where Earth's northern hemisphere leans away from the Sun, it's winter; when the southern hemisphere tilts away, it's winter south of the equator.

While the more massive planets have stable obliquities, they range in size from a nearly perpendicular 3° for Jupiter to about 97° for Uranus, Brunini says.